<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>2.3</title>
</head>

<body>
    <script>
        var a = 1000;

        function f() {
            console.log(a);
            if (false) {
                var a = 'ni hao';
            }
        }
        f();    //undefined
    </script>
</body>

</html>